Gas separation method and zeolite membrane

ABSTRACT

A gas separation method includes supplying a mixed gas to a zeolite membrane complex and permeating a high permeability gas through the zeolite membrane complex to separate the high permeability gas from other gases. The mixed gas includes a high permeability gas and a trace gas that is lower in concentration than the high permeability gas. The molar concentration of a first gas included in the trace gas in the mixed gas is higher than the molar concentration of a second gas included in the trace gas in the mixed gas. The adsorption equilibrium constant of the first gas on the zeolite membrane is less than 60 times that of the high permeability gas. The adsorption equilibrium constant of the second gas on the zeolite membrane is 400 times or more that of the high permeability gas.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of InternationalApplication No. PCT/JP2021/005278 filed on Feb. 12, 2021, which claimsthe benefit of priority to Japanese Patent Application No. 2020-047802filed on Mar. 18, 2020. The entire contents of these applications areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a gas separation method of separating amixed gas, and a zeolite membrane formed on a porous support.

BACKGROUND ART

At present, there is demand for the ability to separate a specificcomponent from a combustion exhaust gas produced in a chemical plant orany other facility. For example, Japanese Patent Application Laid-OpenNo. 2012-236123 (Document 1) proposes a system for separating andcollecting carbon dioxide in an exhaust gas by permeating carbon dioxidethrough a zeolite membrane. International Publication No. WO2016/052058(Document 2) proposes a technique for separating an olefin compound froma fluid to be processed, using a zeolite membrane.

The separation of carbon dioxide in an exhaust gas, as described inDocument 1, may cause a time-varying reduction in selectivity due tominor components contained in the exhaust gas being adsorbed on smallpores of the zeolite membrane (see Ines Tiscornia et.al., “Separation ofpropylene/propane mixtures by titanosilicate ETS-10 membranes preparedin one-step seeded hydrothera synthesis,” Journal of Membrane Science311 (2008), p. 326-335 (Document 3)). In the case of separating carbondioxide or the like from an exhaust gas produced in a plant such as anolefin production plant, a synthetic alcohol production plant, or anester production plant, the minor components may, for example, beolefin, alcohol, ester, or carboxylic acid that is an end product, anunreacted raw material, or an intermediate product.

Document 1 proposes to guide the exhaust gas to a pretreatment facilitybefore the exhaust gas is supplied to the zeolite membrane and to removemoisture in the exhaust gas in the pretreatment facility to reducemoisture content. Meanwhile, Document 2 describes performing processingfor reducing a compound of acetylene series, processing for reducing asulfur compound, or processing for reducing a fine particle compositionin a pre-treatment unit.

Incidentally, since the variety of minor substances contained in theexhaust gas or any other gas is wide, the pretreatment facility may beupsized or may become complicated if all of these minor substances areto be removed in the pretreatment facility.

SUMMARY OF THE INVENTION

The present invention is intended for a gas separation method ofseparating a mixed gas, and it is an object of the present invention toeasily suppress a time-varying reduction in permeability of a zeolitemembrane.

A gas separation method according to a preferable embodiment of thepresent invention includes a) preparing a zeolite membrane complex thatincludes a porous support and a zeolite membrane formed on the support,and b) supplying a mixed gas that includes a high permeability gas and atrace gas to the zeolite membrane complex, the trace gas being lower inconcentration than the high permeability gas, and permeating the highpermeability gas through the zeolite membrane complex to separate thehigh permeability gas from other gases. A molar concentration of a firstgas included in the trace gas in the mixed gas is higher than a molarconcentration of a second gas included in the trace gas in the mixedgas, an adsorption equilibrium constant of the first gas on the zeolitemembrane being less than 60 times an adsorption equilibrium constant ofthe high permeability gas on the zeolite membrane, and an adsorptionequilibrium constant of the second gas on the zeolite membrane being 400times or more the adsorption equilibrium constant of the highpermeability gas in the zeolite membrane.

Accordingly, it is possible to easily suppress a time-varying reductionin permeability of the zeolite membrane.

Preferably, the molar concentration of the first gas in the mixed gas is40 times or more the molar concentration of the second gas in the mixedgas.

Preferably, the high permeability gas is hydrogen, nitrogen, oxygen, orcarbon dioxide.

Preferably, at least one of the first gas and the second gas is anorganic substance.

Preferably, the zeolite membrane contains a zeolite crystal whosemaximum number of membered rings is 8.

Preferably, the gas separation method described above further includes,before the operation b), removing the second gas from the mixed gas.

Preferably, the mixed gas further includes a low permeability gas towhich the zeolite membrane has lower permeability than to the highpermeability gas, and in the mixed gas, the trace gas is lower inconcentration than the low permeability gas.

Preferably, the mixed gas contains one or more types of substancesselected from among hydrogen, helium, nitrogen, oxygen, water, watervapor, carbon monoxide, carbon dioxide, nitrogen oxide, ammonia, sulfuroxide, hydrogen sulfide, sulfur fluoride, mercury, arsine, hydrogencyanide, carbonyl sulfide, C1 to C8 hydrocarbons, organic acid, alcohol,mercaptans, ester, ether, ketone, and aldehyde.

The present invention is also directed to a zeolite membrane formed on aporous support. In the zeolite membrane described above, when a mixedgas that includes a high permeability gas and a trace gas that is lowerin concentration than the high permeability gas is supplied to thezeolite membrane, and in a state in which a molar concentration of afirst gas included in the trace gas in the mixed gas is higher than amolar concentration of a second gas included in the trace gas in themixed gas, a permeance ratio of P40/P10 is higher than or equal to 0.77and lower than or equal to 1.00, an adsorption equilibrium constant ofthe first gas on the zeolite membrane being less than 60 times anadsorption equilibrium constant of the high permeability gas on thezeolite membrane, and an adsorption equilibrium constant of the secondgas on the zeolite membrane being 400 times or more the adsorptionequilibrium constant of the high permeability gas on the zeolitemembrane.

Accordingly, it is possible to easily suppress a time-varying reductionin permeability of the zeolite membrane.

Preferably, the high permeability gas is hydrogen, nitrogen, oxygen, orcarbon dioxide.

Preferably, the mixed gas further includes a low permeability gas towhich the zeolite membrane has lower permeability than to the highpermeability gas, and in the mixed gas, the trace gas is lower inconcentration the low permeability gas.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of a separation device;

FIG. 2 is a sectional view of a zeolite membrane complex;

FIG. 3 is a sectional view showing part of the zeolite membrane complexin enlarged dimensions; and

FIG. 4 is a flowchart for the separation of a mixed gas.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is an illustration of a general structure of a separation device2 according to one embodiment of the present invention. The separationdevice 2 is a device that supplies a mixed gas including a plurality oftypes of gases to a zeolite membrane complex 1 and permeates a highpermeability gas included in the mixed gas through the zeolite membranecomplex 1 so as to separate the high permeability gas from the mixedgas. For example, the separation conducted by the separation device 2may be for the purpose of extracting the high permeability gas from themixed gas, or for the purpose of concentrating a low permeability gas.

For example, the mixed gas may contain one or more types of substancesselected from among hydrogen (H₂), helium (He), nitrogen (N₂), oxygen(O₂), water (H₂O), water vapor (H₂O), carbon monoxide (CO), carbondioxide (CO₂), nitrogen oxides, ammonia (NH₃), sulfur oxides, hydrogensulfide (H₂S), sulfur fluorides, mercury (Hg), arsine (AsH₃), hydrogencyanide (HCN), carbonyl sulfide (COS), C1 to C8 hydrocarbons, organicacid, alcohol, mercaptans, ester, ether, ketone, and aldehyde.

Nitrogen oxides are compounds of nitrogen and oxygen. For example, theaforementioned nitrogen oxides may be gases called NOx such as nitrogenmonoxide (NO), nitrogen dioxide (NO₂), nitrous oxide (also referred toas dinitrogen monoxide) (N₂O), dinitrogen trioxide (N₂O₃), dinitrogentetroxide (N₂O₄), or dinitrogen pentoxide (N₂O₅).

Sulfur oxides are compounds of sulfur and oxygen. For example, theaforementioned sulfur oxides may be gases called SO_(x) such as sulfurdioxide (SO₂) or sulfur trioxide (SO₃).

Sulfur fluorides are compounds of fluorine and sulfur. For example, theaforementioned sulfur fluorides may be disulfur difluoride (F—S—S—F,S═SF₂), sulfur difluoride (SF₂), sulfur tetrafluoride (SF₄), sulfurhexafluoride (SF₆), or disulfur decafluoride (S₂F₁₀).

C1 to C8 hydrocarbons are hydrocarbons that contain one or more andeight or less carbon atoms. C3 to C8 hydrocarbons each may any of alinear-chain compound, a side-chain compound, and a cyclic compound. C2to C8 hydrocarbons each may be either of a saturated hydrocarbon (i.e.,where double bonds and triple bonds are not located in molecules) and anunsaturated hydrocarbon (i.e., where double bonds and/or triple bondsare located in molecules). C1 to C4 hydrocarbons may, for example, bemethane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), propane (C₃H₈), propylene(C₃H₆), normal butane (CH₃(CH₂)₂CH₃), isobutene (CH(CH₃)₃), 1-butene(CH₂=CHCH₂CH₃), 2-butene (CH₃CH═CHCH₃), or isobutene (CH₂=C(CH₃)₂).

The aforementioned organic acid may, for example, be carboxylic acid orsulfonic acid. For example, carboxylic acid may be formic acid (CH₂O₂),acetic acid (C₂H₄O₂), oxalic acid (C₂H₂O₄), acrylic acid (C₃H₄O₂), orbenzoic acid (C₆H₅COOH). Sulfonic acid may, for example, be ethanesulfonic acid (C₂H₆O₃S). The organic acid may be a chain compound, ormay be a cyclic compound.

The aforementioned alcohol may, for example, be methanol (CH₃OH),ethanol (C₂H₅OH), isopropanol (2-propanol) (CH₃CH(OH)CH₃), ethyleneglycol (CH₂(OH)CH₂(OH)), or butanol (C₄H₉OH).

Mercaptans are organic substances with terminal sulfur hydrides (SH) andare substances called also thiol or thioalcohol. The aforementionedmercaptans may, for example, be methyl mercaptan (CH₃SH), ethylmercaptan (C₂H₅SH), or 1-propane thiol (C₃H₇SH).

The aforementioned ester may, for example, be formic acid ester oracetic acid ester.

The aforementioned ether may, for example, be dimethyl ether ((CH₃)₂O ),methyl ethyl ether (C₂H₅OCH3), or diethyl ether ((C₂H₅)₂O).

The aforementioned ketone may, for example, be acetone ((CH₃)₂CO),methyl ethyl ketone (C₂H₅COCH₃), or diethyl ketone ((C₂H₅)₂CO).

The aforementioned aldehyde may, for example, be acetaldehyde (CH₃CHO),propionaldehyde (C₂H₅CHO), or butanal (butyraldehyde) (C₃H₇CHO).

The separation device 2 includes the zeolite membrane complex 1, asealer 21, an outer cylinder 22, two seal members 23, a supplier 26, afirst collector 27, and a second collector 28. The zeolite membranecomplex 1, the sealer 21, and the seal members 23 are placed in theinternal space of the outer cylinder 22. The supplier 26, the firstcollector 27, and the second collector 28 are arranged outside the outercylinder 22 and connected to the outer cylinder 22. FIG. 1 omits anillustration of cross hatching in the section of part of theconfiguration.

The outer cylinder 22 is a generally cylindrical tube-like member. Forexample, the outer cylinder 22 may be formed of stainless steel orcarbon steel. The longitudinal direction of the outer cylinder 22 isapproximately parallel to the longitudinal direction of the zeolitemembrane complex 1. One end of the outer cylinder 22 in the longitudinaldirection (i.e., the end on the left side in FIG. 1 ) has a supply port221, and the other end thereof has a first exhaust port 222. The sideface of the outer cylinder 22 has a second exhaust port 223. The supplyport 221 is connected to the supplier 26. The first exhaust port 222 isconnected to the first collector 27. The second exhaust port 223 isconnected to the second collector 28. The internal space of the outercylinder 22 is an enclosed space isolated from the space around theouter cylinder 22.

The supplier 26 supplies a mixed gas to the internal space of the outercylinder 22 via the supply port 221. For example, the supplier 26 may bea blower or a pump that pumps the mixed gas toward the outer cylinder22. The blower or the pump includes a pressure regulator that controlsthe pressure of the mixed gas supplied to the outer cylinder 22. Thefirst collector 27 and the second collector 28 may, for example, bereservoirs that store a gas delivered from the outer cylinder 22, or maybe blowers or pumps that transfer the above gas.

The sealer 21 is a member that is attached to both ends of the support11 in the longitudinal direction (i.e., in the left-right direction inFIG. 1 ) to cover and seal both end faces of the support 11 in thelongitudinal direction and the outside surface of the support 11 in thevicinity of the both end faces. The sealer 21 prevents the inflow andoutflow of gas from the both end faces of the support 11. For example,the sealer 21 may be a plate-like member formed of glass or resin. Thematerial and shape of the sealer 21 may be appropriately changed. Notethat the sealer 21 has a plurality of openings that overlap with aplurality of through holes 111 (described later) of the support 11, sothat both ends of each through hole 111 of the support 11 in thelongitudinal direction are not covered with the sealer 21. This allowsthe inflow and outflow of gas or the like from the both ends into andout of the through holes 111.

The two seal members 23 are arranged around the entire circumferencebetween the outside surface of the zeolite membrane complex 1 and theinside surface of the outer cylinder 22 in the vicinity of the both endsof the zeolite membrane complex 1 in the longitudinal direction. Eachseal member 23 is an approximately ring-shaped member formed of amaterial that is impermeable to gas. For example, the seal members 23may be O-rings formed of resin having flexibility. The seal members 23are in tight contact with the outside surface of the zeolite membranecomplex 1 and the inside surface of the outer cylinder 22 along theentire circumference. In the example illustrated in FIG. 1 , the sealmembers 23 are in tight contact with the outside surface of the sealer21 and indirectly in tight contact with the outside surface of thezeolite membrane complex 1 via the sealer 21. The space between the sealmembers 23 and the outside surface of the zeolite membrane complex 1 andthe space between the seal members 23 and the inside surface of theouter cylinder 22 are sealed so as to almost or completely disable thepassage of gas.

FIG. 2 is a sectional view of the zeolite membrane complex 1. FIG. 3 isa sectional view showing part of the zeolite membrane complex 1 inenlarged dimensions. The zeolite membrane complex 1 includes a poroussupport 11 and a zeolite membrane 12 formed on the support 11. Thezeolite membrane 12 refers to at least a zeolite formed into a membraneon the surface of the support 11, and does not include zeolite particlesthat are merely dispersed in an organic membrane. The zeolite membrane12 may contain two or more types of zeolites having different structuresor different compositions. In FIG. 2 , the zeolite membrane 12 isillustrated with bold lines. In FIG. 3 , the zeolite membrane 12 iscross-hatched. In the illustration of FIG. 3 , the zeolite membrane 12has a thickness greater than its actual thickness.

The support 11 is a porous member that is permeable to gas. In theexample illustrated in FIG. 1 , the support 11 is a monolith support inwhich an integrally-molded column-like body has the plurality of throughholes 111, each extending in the longitudinal direction (i.e., in theright-left direction in FIG. 2 ). In the example illustrated in FIG. 1 ,the support 11 has an approximately column-like shape. Each through hole111 (i.e., cell) may have, for example, an approximately circularcross-sectional shape perpendicular to the longitudinal direction. Inthe illustration of FIG. 1 , the through holes 111 have a diametergreater than its actual diameter, and the number of through holes 111 issmaller than the actual number of through holes 111. The zeolitemembrane 12 is formed on the inside surfaces of the through holes 111and covers approximately the entire inside surfaces of the through holes111.

The support 11 has a length (i.e., length in the left-right direction inFIG. 1 ) of, for example, 10 cm to 200 cm. The support 11 has an outerdiameter of, for example, 0.5 cm to 30 cm. The distance between thecentral axes of each pair of adjacent through holes 111 may, forexample, be in the range of 0.3 mm to 10 mm. The surface roughness (Ra)of the support 11 may, for example, be in the range of 0.1 μm to 5.0 μmand preferably in the range of 0.2 μm to 2.0 μm. Alternatively, thesupport 11 may have a different shape such as a honeycomb shape, a flatplate-like shape, a tube-like shape, a cylinder-like shape, acolumn-like shape, or a polygonal prism shape. In the case where thesupport 11 has a tube- or cylinder-like shape, the support 11 may have athickness of, for example, 0.1 mm to 10 mm.

The material for the support 11 may be any of a variety of substances(e.g., ceramic or metal) as long as the substance has chemical stabilityin the step of forming the zeolite membrane 12 on the surface. In thepresent embodiment, the support 11 is formed of a ceramic sinteredcompact. Examples of the ceramic sintered compact to be selected as thematerial for the support 11 include alumina, silica, mullite, zirconia,titania, yttria, silicon nitride, and silicon carbide. In the presentembodiment, the support 11 contains at least one type of alumina,silica, and mullite.

The support 11 may include an inorganic binder. The inorganic binder maybe at least one of titania, mullite, easily sinterable alumina, silica,glass frit, clay minerals, and easily sinterable cordierite.

The support 11 may have a mean pore diameter of, for example, 0.01 μm to70 μm and preferably 0.05 μm to 25 μm. The mean pore diameter of thesupport 11 in the vicinity of the surface on which the zeolite membrane12 is formed is in the range of 0.01 μm to 1 μm and preferably in therange of 0.05 μm to 0.5 μm. The mean pore diameter may be measured by,for example, a mercury porosimeter, a perm porosimeter, or a nano-permporosimeter. Referring to a pore size distribution of the support 11 asa whole including the surface and inside of the support 11, D5 may be inthe range of, for example, 0.01 μm to 50 μm, D50 may be in the range of,for example, 0.05 μm to 70 μm, and D95 may be in the range of, forexample, 0.1 μm to 2000 μm. The porosity of the support 11 in thevicinity of the surface on which the zeolite membrane 12 is formed maybe in the range of, for example, 20% to 60%.

For example, the support 11 may have a multilayer structure in which aplurality of layers having different mean pore diameters are laminatedone above another in the thickness direction. A surface layer thatincludes the surface on which the zeolite membrane 12 is formed has asmaller mean pore diameter and a smaller sintered particle diameter thanthe other layers (other than the surface layer). The mean pore diameterof the surface layer of the support 11 may be in the range of, forexample, 0.01 μm to 1 μm and preferably in the range of 0.05 μm to 0.5μm. In the case where the support 11 has a multilayer structure, thematerial for each layer may be any of the substances described above.The materials for the plurality of layers forming the multilayerstructure may be the same, or may be different.

The zeolite membrane 12 is a porous membrane with small pores. Thezeolite membrane 12 may be used as a separation membrane that separatesa specific substance from a mixed gas including a plurality of types ofgases, using a molecular-sieving function. The zeolite membrane 12 isless permeable to other gases than to a specific gas. In other words,the permeance of the zeolite membrane 12 to other gases is lower thanthe permeance of the zeolite membrane 12 to the specific gas describedabove.

The zeolite membrane 12 may have a thickness of, for example, 0.05 μm to30 μm, preferably 0.1 μm to 20 μm, and more preferably 0.5 μm to 10 μm.Increasing the thickness of the zeolite membrane 12 improvesselectivity. Reducing the thickness of the zeolite membrane 12 increasespermeance. The surface roughness (Ra) of the zeolite membrane 12 may,for example, be less than or equal to 5 μm, preferably less than orequal to 2 μm, more preferably less than or equal to 1 μm, and yet morepreferably less than or equal to 0.5 μm.

The mean pore diameter of the zeolite membrane 12 may be preferablygreater than or equal to 0.2 nm and less than or equal to 0.8 nm, morepreferably greater than or equal to 0.3 nm and less than or equal to 0.5nm, and yet more preferably greater than or equal to 0.3 nm and lessthan or equal to 0.4 nm. The mean pore diameter of the zeolite membrane12 is smaller than the mean pore diameter of the support 11 in thevicinity of the surface where the zeolite membrane 12 is formed.

The mean pore diameter is assumed to be an arithmetical mean of themajor and minor axes of an n-membered ring pore, where n is the maximumnumber of membered rings in the zeolite of the zeolite membrane 12. Inthe case where the zeolite has a plurality of n-membered ring poreswhere n is the same number, the mean pore diameter is assumed to be anarithmetical mean of the major and minor axes of all of the n-memberedring pores. Note that the n-membered ring refers to a portion in whichthe number of oxygen atoms constituting the framework of a pore is n andthat forms a cyclic structure as a result of each oxygen atom beingbonded to a T atom described later. The n-membered ring also refers to aportion that forms a through hole (channel), and does not include aportion that fails to form a through hole. The n-membered ring porerefers to a small pore formed of an n-membered ring.

The mean pore diameter of the zeolite membrane is uniquely determined bythe framework structure of the zeolite and can be obtained from a valuedisclosed in the “Database of Zeolite Structures” by the InternationalZeolite Association, [online], from the Internet<URL:http://www.iza-structure.org/databases/>.

There are no particular limitations on the type of the zeolite of thezeolite membrane 12, and examples of the zeolite include AEI-, AEN-,AFN-, AFV-, AFX-, BEA-, CHA-, DDR-, ERI-, ETL-, FAU- (X-type, Y-type),GIS-, LEV-, LTA-, MEL-, MFI-, MOR-, PAU-, RHO-, SAT-, and SOD-typezeolites. More preferable examples of the zeolite include AEI-, AFN-,AFV-, AFX-, CHA-, DDR-, ERI-, ETL-, GIS-, LEV-, LTA-, PAU-, RHO-, andSAT-type zeolites. Yet more preferable examples of the zeolite includeAEI-, AFN-, AFV-, AFX-, CHA-, DDR-, ERI-, ETL-, GIS-, LEV-, PAU-, RHO-,and SAT-type zeolites.

The zeolite of the zeolite membrane 12 may contain, for example, Al as Tatoms. The zeolite of the zeolite membrane 12 may, for example, be azeolite that contains silicon (Si) and aluminum (Al) as atoms (T atoms)located in the center of an oxygen tetrahedron (TO₄) constituting thezeolite, an AIPO-type zeolite that contains Al and phosphorus (P) as theT atoms, an SAPO-type zeolite that contains Si, Al, and P as the Tatoms, an MAPSO-type zeolite that contains magnesium (Mg), Si, Al, and Pas the T atoms, or a ZnAPSO-type zeolite that contains zinc (Zn), Si,Al, and P as the T atoms. Some of the T atoms may be replaced by otherelements.

The zeolite membrane 12 may contain, for example, Si. For example, thezeolite membrane 12 may contain any two or more of Si, Al, and P. Thezeolite membrane 12 may contain alkali metal. The alkali metal may, forexample, be sodium (Na) or potassium (K). In the case where the zeolitemembrane 12 contains Si atoms, the Si/Al ratio in the zeolite membrane12 may, for example, be higher than or equal to one and lower than orequal to a hundred thousand. The Si/Al ratio is preferably higher thanor equal to 5, more preferably higher than or equal to 20, and yet morepreferably higher than or equal to 100. A higher Si/Al ratio is morepreferable. The Si/AI ratio in the zeolite membrane 12 may be adjustedby adjusting, for example, the compounding ratio of an Si source and anAl source in a starting material solution described later.

From the viewpoint of increasing CO₂ permeance and improvingselectivity, it is preferable that the maximum number of membered ringsin the zeolite is 8. For example, the zeolite membrane 12 may be aDDR-type zeolite. In other words, the zeolite membrane 12 is a zeolitemembrane composed of a zeolite having a framework type code “DDR”assigned by the International Zeolite Association. In this case, theintrinsic pore diameter of the zeolite of the zeolite membrane 12 is0.36 nm×0.44 nm and the mean pore diameter thereof is 0.40 nm.

The CO₂ permeance of the zeolite membrane 12 at a temperature of 20° C.to 400° C. may, for example, be higher than or equal to 100nmol/m²·s·Pa. The ratio (permeance ratio) between CO₂ permeance and CH₄leakage (permeance) of the zeolite membrane 12 at a temperature of 20°C. to 400° C. may, for example, be higher than or equal to 100. The CO₂permeance and the permeance ratio are values for the case where adifference in CO₂ partial pressure between the supply side andpermeation side of the zeolite membrane 12 is 1.5 MPa.

Next, one example of the procedure for producing the zeolite membranecomplex 1 will be described. In the production of the zeolite membranecomplex 1, first, seed crystals for use in the production of the zeolitemembrane 12 are prepared. For example, the seed crystals may be acquiredfrom DDR-type zeolite powder synthesized by hydrothermal synthesis. Thiszeolite powder may be used as-is as the seed crystals, or may beprocessed into the seed crystals by pulverization or any other method.

Then, the porous support 11 is immersed in a solution in which the seedcrystals are dispersed, so as to cause the seed crystals to adhere tothe support 11. Alternatively, a solution in which the seed crystals aredispersed may be brought into contact with a portion of the surface ofthe support 11 on which the zeolite membrane 12 is desired to be formed,so as to cause the seed crystals to adhere to the support 11. In thisway, a seed-crystal-deposited support is prepared. The seed crystals maybe caused to adhere to the support 11 by any other method.

The support 11 with the seed crystals adhering thereto is immersed in astarting material solution. The starting material solution may beprepared by, for example, dissolving substances such as an Si source anda structure-directing agent (hereinafter, also referred to as an “SDA”)in a solvent. The composition of the starting material solution may, forexample, be 1.0 SiO₂: 0.015 SDA: 0.12 (CH₂)₂(NH₂)₂. The solvent in thestarting material solution may, for example, be water or alcohol such asethanol. The SDA contained in the starting material solution may, forexample, be an organic substance. The SDA may, for example, be1-adamantanamine.

Then, the DDR-type zeolite is grown by hydrothermal synthesis using theseed crystals as nuclei, so as to form the DDR-type zeolite membrane 12on the support 11. Preferably, the temperature of the hydrothermalsynthesis may be in the range of 120 to 200° C., and may for example, be160° C. Preferably, the hydrothermal synthesis time may be in the rangeof 10 to 100 hours and may, for example, be 30 hours.

After the completion of the hydrothermal synthesis, the support 11 andthe zeolite membrane 12 are rinsed with deionized water. After therinsing, the support 11 and the zeolite membrane 12 are dried at, forexample, 80° C. After the support 11 and the zeolite membrane 12 havebeen dried, the zeolite membrane 12 is subjected to heat treatment so asto almost completely burn and remove the SDA in the zeolite membrane 12and cause micropores in the zeolite membrane 12 to come through thezeolite membrane 12. In this way, the aforementioned zeolite membranecomplex 1 is obtained.

Next, the procedure for separating a mixed gas, performed by theseparation device 2, will be described with reference to FIG. 4 . In thecase of separating a mixed gas, the zeolite membrane complex 1 is firstprepared by preparing the aforementioned separation device 2 (step S11).

Then, the supplier 26 supplies a mixed gas to the internal space of theouter cylinder 22, the mixed gas including a plurality of types of gasesto which the zeolite membrane 12 has different permeability. The mixedgas includes a high permeability gas and a trace gas. The mixed gas mayfurther include a low permeability gas. Hereinafter, a case is describedin which the mixed gas includes a high permeability gas, a lowpermeability gas, and a trace gas. The trace gas is a component whosemolar concentration (mol %) in the mixed gas is less than 10 mol %. Thetrace gas may be one type of gas, or may include two or more types ofgas. In the case where the mixed gas includes two or more types of tracegas, the molar concentration of each type of trace gas in the mixed gasis less than 10 mol %, and a total molar concentration of the two ormore types of trace gas in the mixed gas may be higher than 10 mol %.For example, the total molar concentration may be in the range of 1 mol% to 30 mol % and preferably in the range of 1 mol % to 10 mol %.

The high permeability gas and the low permeability gas are principalcomponents of the mixed gas (i.e., components excluding the trace gasfrom the mixed gas). Among the principal components of the mixed gas,the high permeability gas is one type of gas to which the zeolitemembrane 12 has highest permeability. The low permeability gas is acomponent that is obtained by excluding the high permeability gas fromthe principal components of the mixed gas and to which the zeolitemembrane 12 has lower permeability than to the high permeability gas.The low permeability gas may be one type of gas or two or more types ofgas. In other words, the mixed gas may include three or more types ofgas as its principal components. The molar concentration of the highpermeability gas in the mixed gas and the molar concentration of the lowpermeability gas in the mixed gas (if there are two or more types ofgas, each type of low permeability gas) are higher than or equal to 10mol %. The molar concentration of the trace gas in the mixed gas islower than the molar concentration of the high permeability gas in themixed gas and the molar concentration of the low permeability gas in themixed gas (if there are two or more types of gas, each type of lowpermeability gas). A total content of the high permeability gas and thelow permeability gas (i.e., the content of the principle components) inthe mixed gas may be in the range of, for example, 70 mol % to 99 mol %and preferably in the range of 90 mol % to 99 mol %. The highpermeability gas may, for example, be H₂, N₂, O₂, or CO₂. The lowpermeability gas may contain, for example, one or more types of N₂, air,and CH₄. In the present embodiment, the high permeability gas and thelow permeability gas are CO₂ and CH₄, respectively.

The trace gas includes a first gas and a second gas. The first gas is alow adsorbing gas whose adsorption equilibrium constant on the zeolitemembrane 12 is less than 60 times the adsorption equilibrium constant ofthe high permeability gas on the zeolite membrane 12. It is preferablethat the adsorption equilibrium constant of the first gas on the zeolitemembrane 12 is more than one times larger than the adsorptionequilibrium constant of the high permeability gas on the zeolitemembrane 12. The second gas is a high adsorbing gas whose adsorptionequilibrium constant on the zeolite membrane 12 is 400 times or more theadsorption equilibrium constant of the high permeability gas on thezeolite membrane 12. A higher adsorption equilibrium constant relativelyincreases the tendency of the gas to be adsorbed on small pores of thezeolite membrane 12, and a lower adsorption equilibrium constantrelatively reduces the tendency of the gas to be adsorbed on small poresof the zeolite membrane 12.

The adsorption equilibrium constant of the high permeability gas on thezeolite membrane 12 can be calculated by, for example, conducting anadsorption test on the adsorption of the high permeability gas on thezeolite membrane 12 and creating a Langmuir plot from obtainedmeasurement data, i.e., an adsorption isotherm. Specifically, zeolitepowder of the same type as the zeolite membrane 12 (i.e., DDR-typezeolite powder) is first prepared. The Si/Al ratio in the zeolite powderis substantially the same as the Si/Al ratio in the zeolite membrane 12.Moreover, a glass container for the adsorption test is prepared, and theweight of the empty glass container is measured. Then, the zeolitepowder with a predetermined weight is placed in the glass container.Then, the zeolite powder is heated and the glass container is evacuatedin order to obtain vacuum atmosphere (i.e., produce a vacuum).Accordingly, adsorbents are desorbed from the zeolite powder.Thereafter, the weight of the glass container that contains the zeolitepowder is measured, and the aforementioned weight of the empty glasscontainer is subtracted from this measured weight to obtain the weightof the zeolite powder.

Processing described below is conducted in a state in which thetemperature of a portion of the glass container in which the zeolitepowder is placed is maintained at a predetermined temperature. Thepredetermined temperature is the same as the temperature at which theseparation device 2 separates the mixed gas, and may, for example, beambient temperature (25° C.). A predetermined amount of highpermeability gas is introduced into the glass container in a vacuumatmosphere. In the glass container, some of the high permeability gas isadsorbed on the zeolite powder. This lowers the pressure inside theglass container. Upon confirmation of the fact that the pressure insidethe glass container becomes constant and the adsorption of the highpermeability gas on the zeolite powder becomes stabilized, pressure Pinside the glass container is measured, and the amount of adsorption qof the high permeability gas is calculated, using the amount ofreduction in pressure caused by the adsorption. Thereafter, theintroduction of the high permeability gas into the glass container andthe acquisition of the pressure P and the amount of adsorption q arerepeated in the same manner as described above.

Then, the measured values of the pressure P and the amount of adsorptionq are plotted, where P is the horizontal axis and P/q is the verticalaxis. Then, the plotted points are subjected to collinear approximationby the least-squares method or any other method, so as to obtain aninclination a and an intercept b of an approximate straight line. Then,the adsorption equilibrium constant of the high permeability gas iscalculated by dividing the inclination a by the intercept b. Theadsorption equilibrium constants of the high adsorbing gas and the lowadsorbing gas are also calculated by the same method described above.

The molar concentration of the first gas in the mixed gas is higher thanthe molar concentration of the second gas in the mixed gas. It ispreferable that the molar concentration of the first gas in the mixedgas is 40 times or more the molar concentration of the second gas in themixed gas. The molar concentration of the first gas in the mixed gas maybe in the range of, for example, 1 mol % to 30 mol %, and preferably inthe range of 1 mol % to 10 mol %. Note that the molar concentration ofthe first gas in the mixed gas may be higher than 30 mol %, but whenfactors such as cost and workability are taken into consideration, themolar concentration of the first gas in the mixed gas is preferablylower than or equal to 30 mol %. The molar concentration of the secondgas in the mixed gas may be in the range of, for example, 0 mol % to 2mol %, and preferably in the range of 0 mol % to 0.25 mol %.

The first gas may be one type of gas, or may include a plurality oftypes of gas. In the case where the first gas includes a plurality oftypes of gas, the molar concentration of the first gas in the mixed gasis a total molar concentration of these types of gas. The second gas maybe one type of gas, or may include a plurality of types of gas. In thecase where the second gas includes a plurality of types of gas, themolar concentration of the second gas in the mixed gas is a total molarconcentration of these types of gas.

The first gas and the second gas each may be a gas of organic substance,may be a gas of inorganic substance, or may be a mixed gas of organicand inorganic substances. For example, at least one of the first gas andthe second gas may be an organic substance. In the present embodiment,both of the first gas and the second gas are organic substances. Forexample, the first gas may be an organic substance that contains carbon(C) and hydrogen (H) or an organic substance that contains carbon (C),hydrogen (H), and oxygen (O) and may be a gas with a vapor pressure morethan or equal to 50 kPa or more at 25° C. For example, the first gas maybe ethylene or propylene. The second gas may, for example, be an organicsubstance that contains carbon (C) and hydrogen (H) or an organicsubstance that contains carbon (C), hydrogen (H), and oxygen (O) and maybe a gas with a vapor pressure less than 30 kPa at 25° C. For example,the second gas may be vinyl acetate or ethanol. The trace gas mayfurther include a different gas other than the first gas and the secondgas. Alternatively, the trace gas may substantially not include thesecond gas. In this case, the molar concentration of the second gas inthe mixed gas is substantially 0 mol %.

In the separation method illustrated by way of example in FIG. 4 , thesecond gas is removed from the mixed gas before the mixed gas issupplied to the separation device 2 (step S12). This lowers the molarconcentration of the second gas in the mixed gas. The removal of thesecond gas in step S12 may be implemented by, for example, passing themixed gas through a liquid that has high absorbency for the second gasso as to cause the second gas to be absorbed by the liquid.Alternatively, the removal of the second gas may be implemented bypassing the mixed gas through an adsorbent that has high adsorptivityfor the second gas so as to cause the second gas to be adsorbed on theadsorbent. Note that the processing for removing the second gas in stepS12 needs only to be capable of lowering the molar concentration of thesecond gas in the mixed gas, and there is no need to remove the entirevolume of the second gas in the mixed gas.

Note that step S12 intentionally does not include removal of the firstgas from the mixed gas. In other words, processing intended to removethe first gas from the mixed gas is not performed in step S12. In thisway, in step S12, the second gas, which is the high adsorbing gas, isselectively removed from the trace gas included in the mixed gas. Thissimplifies the pretreatment performed before the separation of the highpermeability gas in step S13 described later. Note that step S12 mayalso include the removal of the first gas.

After step S12 ends, the mixed gas with the second gas removed therefromis supplied from the supplier 26 to the outer cylinder 22. The pressureof the mixed gas supplied from the supplier 26 to the internal space ofthe outer cylinder 22 (i.e., initial pressure) may be in the range of,for example, 0.1 MPa to 20.0 MPa. The temperature at which the mixed gasis separated may be in the range of, for example, 10° C. to 200° C. Themixed gas supplied from the supplier 26 to the outer cylinder 22 isintroduced from the left end of the zeolite membrane complex 1 in thedrawing into each through hole 111 of the support 11 as indicated by anarrow 251. The high permeability gas (e.g., CO₂) in the mixed gaspermeates through the zeolite membrane 12 provided on the inside surfaceof each through hole 111 and through the support 11 and is exhaustedfrom the outside surface of the support 11. Accordingly, the highpermeability gas is separated from other gases such as the lowpermeability gas (e.g., CH₄) in the mixed gas (step S13). The gasexhausted from the outside surface of the support 11 (hereinafter,referred to as the “permeated gas”) is collected by the second collector28 via the second exhaust port 223 as indicated by an arrow 253. Thepressure of the gas collected by the second collector 28 via the secondexhaust port 223 (i.e., permeation pressure) may, for example, beapproximately 1 atmospheric pressure (0.101 MPa).

In the mixed gas, the gas (hereinafter, referred to as “non-permeatedgas”) other than the gas that has permeated through the zeolite membrane12 and the support 11 passes through each through hole 111 of thesupport 11 from the left side to the right in FIG. 1 and is collected bythe first collector 27 via the first exhaust port 222 as indicated by anarrow 252. For example, the pressure of the gas collected by the firstcollector 27 via the first exhaust port 222 may be approximately thesame as the initial pressure. The non-permeated gas may include, inaddition to the low permeability gas and the trace gas described above,a high permeability gas that has not permeated through the zeolitemembrane 12.

As described above, in the separation method illustrated by way ofexample in FIG. 4 , the molar concentration of the second gas includedin the trace gas in the mixed gas is low. This suppresses the adsorptionof the second gas (i.e., high adsorbing gas) included in the trace gason small pores of the zeolite membrane 12. Accordingly, it is possibleto reduce the occurrence of a situation in which the permeation of thehigh permeability gas through the zeolite membrane 12 may be inhibitedby the second gas adsorbed on the zeolite membrane 12. As a result, itis possible to suppress a time-varying reduction in permeability of thezeolite membrane 12 to the high permeability gas.

Next, the relationship of the molar concentrations of the first andsecond gases gas in the mixed gas and the time-varying reduction inpermeability of the zeolite membrane 12 to the high permeability gaswill be described with reference to Table 1 and Table 2. The permeanceratio (P40/P10) shown in the table was obtained based on the permeanceof the zeolite membrane complex 1 to gas after the mixed gas includingthe high permeability gas (e.g., CO₂) was supplied to the zeolitemembrane complex 1 in the separation device 2 described above.Specifically, the permeance ratio (P40/P10) was obtained by dividing theaforementioned permeance after a lapse of 40 minutes since the supply ofthe mixed gas was started, by the aforementioned permeance after a lapseof 10 minutes since the supply of the mixed gas was started. Thepressure of the mixed gas supplied from the supplier 26 was assumed tobe 0.4 MPa, and the pressure of the gas collected by the secondcollector 28 (i.e., permeation pressure) was assumed to be approximately1 atmospheric pressure (0.101 MPa). Note that the zeolite membrane 12 ofthe zeolite membrane complex 1 was a DDR-type zeolite.

TABLE 1 High Permeability Gas Low Permeability Gas First Gas Second GasMolar Molar Molar Molar Molar Concentration Concentration ConcentrationConcentration Concentration Ratio First Gas/ Type (mol %) Type (mol %)Type (mol %) Type (mol %) Second Gas Example 1 CO₂ 50 CH₄ 40.975ethylene 9 ethanol 0.025 360 Example 2 CO₂ 50 CH₄ 44.975 ethylene 5ethanol 0.025 200 Example 3 CO₂ 50 CH₄ 48.975 ethylene 1 ethanol 0.02540 Example 4 CO₂ 50 CH₄ 40.975 propylene 9 ethanol 0.025 360 Example 5CO₂ 50 CH₄ 44.975 propylene 5 ethanol 0.025 200 Example 6 CO₂ 50 CH₄48.975 propylene 1 ethanol 0.025 40 Comparative CO₂ 50 CH₄ 49.975 — 0ethanol 0.025 0 Example 1 Example 7 CO₂ 50 CH₄ 40.8 ethylene 9 vinyl 0.245 acetate Example 8 CO₂ 50 CH₄ 40.8 propylene 9 vinyl 0.2 45 acetateExample 9 CO₂ 50 CH₄ 44.8 ethylene 5 vinyl 0.2 25 acetate Example 10 CO₂50 CH₄ 48.8 propylene 1 vinyl 0.2 5 acetate Comparative CO₂ 50 CH₄ 49.8— 0 vinyl 0.2 0 Example 2 acetate

TABLE 2 Adsorption Adsorption Equilibrium Equilibrium Constant ofConstant of First Gas Second Gas Adsorption Adsorption EquilibriumEquilibrium Permeance Constant of High Constant of High RatioPermeability Gas Permeability Gas P40/P10 Example 1 9 474 0.97 Example 29 474 0.93 Example 3 9 474 0.86 Example 4 52 474 0.96 Example 5 52 4740.94 Example 6 52 474 0.88 Comparative — 474 0.76 Example 1 Example 7 9406 0.81 Example 8 52 406 0.77 Example 9 9 406 0.64 Example 10 52 4060.66 Comparative — 406 0.59 Example 2

In Examples 1 to 10 and Comparative Examples 1 and 2, the highpermeability gas and the low permeability gas included in the mixed gaswere respectively CO₂ and CH₄, and the types of the first gas and thesecond gas included in the trace gas were varied. In Examples 1 to 10and Comparative Examples 1 and 2, the molar concentration of the highpermeability gas in the mixed gas was 50 mol %, the molar concentrationof the first gas was varied in the range of 0 mol % to 9 mol %, and themolar concentration of the second gas was varied in the range of 0.025mol % to 0.2 mol %. Note that the molar concentration of the lowpermeability gas in the mixed gas was obtained by subtracting the molarconcentration of the high permeability gas, the molar concentration ofthe first gas, and the molar concentration of the second gas from 100mol %.

In Examples 1 to 3, the first gas was ethylene. The adsorptionequilibrium constant of ethylene on the zeolite membrane 12(hereinafter, also simply referred to as the “adsorption equilibriumconstant”) was nine times the adsorption equilibrium constant of thehigh permeability gas. Thus, the first gas was a low adsorbing gas. Themolar concentration of the first gas in the mixed gas was varied in therange of 1 mol % to 9 mol %. The second gas was ethanol. The adsorptionequilibrium constant of ethanol was 474 times the adsorption equilibriumconstant of the high permeability gas. Thus, the second gas was a highadsorbing gas. The molar concentration of the second gas in the mixedgas was 0.025 mol %. The molar concentration of the first gas in themixed gas was 40 to 360 times the molar concentration of the second gasin the mixed gas. The permeance ratio (P40/P10) was in the range of 0.86to 0.97.

Examples 4 to 6 were the same as Examples 1 to 3, except that the firstgas was changed to propylene. The adsorption equilibrium constant ofpropylene was 52 times the adsorption equilibrium constant of the highpermeability gas. Thus, the first gas was a low adsorbing gas. Thepermeance ratio (P40/P10) was in the range of 0.88 to 0.96.

Comparative Example 1 was the same as Examples 1 to 6, except that thefirst gas as not included in the trace gas (i.e., the molarconcentration of the first gas in the mixed gas was 0 mol %). Thepermeance ratio (P40/P10) was 0.76 and lower than in Examples 1 to 6. Inthis way, in Examples 1 to 6, the inclusion of the first gas in themixed gas makes the permeance ratio higher than in Comparative Example 1and stabilizes the permeability of the zeolite membrane 12.

Examples 7 and 8 were approximately the same as Examples 1 to 6, exceptthat the second gas was changed to vinyl acetate. The adsorptionequilibrium constant of vinyl acetate was 406 times the adsorptionequilibrium constant of the high permeability gas. Thus, the second gaswas a low adsorbing gas. The first gas was ethylene or propylene. Themolar concentration of the first gas in the mixed gas was 9 mol %. Themolar concentration of the second gas in the mixed gas was 0.2 mol %.The molar concentration of the first gas in the mixed gas was 45 timesthe molar concentration of the second gas in the mixed gas. Thepermeance ratio (P40/P10) was in the range of 0.77 to 0.81.

Examples 9 and 10 were the same as Examples 7 and 8, except that themolar concentration of the first gas in the mixed gas was varied in therange of 1 mol % to 5 mol %. The molar concentration of the first gas inthe mixed gas was 5 to 25 times the molar concentration of the secondgas in the mixed gas. The permeance ratio (P40/P10) was in the range of0.64 to 0.66.

Comparative Example 2 was the same as Examples 7 to 10, except that thefirst gas was not included in the trace gas (i.e., the molarconcentration of the first gas in the mixed gas was 0 mol %). Thepermeance ratio (P40/P10) was 0.59 and lower than in Examples 7 to 10.In this way, in Examples 7 to 10, the inclusion of the first gas in themixed gas makes the permeance ratio higher than in Comparative Example 2and stabilizes the permeability of the zeolite membrane 12.

As described above, the gas separation method of separating a mixed gasincludes the step of preparing the zeolite membrane complex 1 (stepS11), and the step of supplying the mixed gas to the zeolite membranecomplex 1 and permeating a high permeability gas through the zeolitemembrane complex 1 to separate the high permeability gas from othergases (step S13). The zeolite membrane complex 1 includes the poroussupport 11 and the zeolite membrane 12 formed on the support 11. Themixed gas includes the high permeability gas and the trace gas that islower in concentration than the high permeability gas. In the trace gas,the molar concentration of the first gas in the mixed gas is higher thanthe molar concentration of the second gas in the mixed gas. Theadsorption equilibrium constant of the first gas on the zeolite membrane12 is less than 60 times the adsorption equilibrium constant of the highpermeability gas. The adsorption equilibrium constant of the second gason the zeolite membrane 12 is 400 times or more the adsorptionequilibrium constant of the high permeability gas.

As described above, in the trace gas included in the mixed gas, themolar concentration of the first gas whose adsorption equilibriumconstant on the zeolite membrane 12 is less than 60 times the adsorptionequilibrium constant of the high permeability gas and is relatively low(i.e., the low adsorbing gas that hardly affects a time-varyingreduction in permeability of the zeolite membrane 12) is made higherthan the molar concentration of the second gas whose adsorptionequilibrium constant on the zeolite membrane 12 is 400 times or more theadsorption equilibrium constant of the high permeability gas and isrelatively high (i.e., the high adsorbing gas). This suppresses theadsorption of the second gas included in the trace gas on small pores ofthe zeolite membrane 12. Accordingly, it is possible to reduce theoccurrence of a situation in which the permeation of the highpermeability gas through the zeolite membrane 12 may be inhibited by thesecond gas adsorbed on the zeolite membrane 12. As a result, it ispossible to suppress a time-varying reduction in permeability of thezeolite membrane 12 to the high permeability gas. Besides, since thereis no need to lower the molar concentration of the first gas to a valueless than or equal to the molar concentration of the second gas, it ispossible to easily suppress a time-varying reduction in permeability.

As described above, it is preferable that the molar concentration of thefirst gas in the mixed gas is 40 times or more the molar concentrationof the second gas in the mixed gas. This further suppresses atime-varying reduction in permeability of the zeolite membrane 12.

As described above, it is preferable that the high permeability gas isH₂, N₂,O₂, or CO₂. In this case, it is possible to efficiently separateH₂, N₂, O₂, or CO₂ while suppressing a time-varying reduction inpermeability.

As described above, it is preferable that the maximum number of memberedrings in a zeolite crystal contained in the zeolite membrane 12 is 8. Inthis case, it is possible to favorably achieve selective permeation of agas with a relatively small molecular size in the zeolite membranecomplex 1.

As described above, it is preferable that at least one of the first gasand the second gas is an organic substance. In this case, it is possibleto easily suppress a time-varying reduction in permeability of thezeolite membrane 12 when the high permeability gas is separated from themixed gas that includes an organic substance as the trace gas.

As described above, it is preferable that the gas separation methodfurther includes the step of removing the second gas from the mixed gas(step S12), the step being performed before step S13. This makes itpossible to easily lower the molar concentration of the second gas inthe mixed gas. As a result, it is possible to easily suppress atime-varying reduction in permeability of the zeolite membrane 12.Besides, in step S12, the second gas (i.e., high adsorbing gas) isselectively removed without intentionally removing the first gas (i.e.,low adsorbing gas). This simplifies the removal step in step S12. As aresult, it is possible to more easily suppress a time-varying reductionin permeability of the zeolite membrane 12.

As described above, it is preferable that the mixed gas further includesa low permeability gas to which the zeolite membrane 12 has lowerpermeability than to the high permeability gas and that theconcentration of the trace gas in the mixed gas is lower than theconcentration of the low permeability gas in the mixed gas. In thiscase, it is possible to achieve a high permeance ratio during theseparation of the high permeability gas from the low permeability gas.

The gas separation method described above is in particular suitable forthe separation of a mixed gas that contains one or more types ofsubstances selected from among hydrogen, helium, nitrogen, oxygen,water, water vapor, carbon monoxide, carbon dioxide, nitrogen oxides,ammonia, sulfur oxides, hydrogen sulfides, sulfur fluorides, mercury,arsine, hydrogen cyanide, carbonyl sulfide, C₁ to C₈ hydrocarbons,organic acid, alcohol, mercaptans, ester, ether, ketone, and aldehyde.

As described above, when the aforementioned mixed gas is supplied to thezeolite membrane 12 formed on the porous support 11 and in a state inwhich the molar concentration of the first gas included in the trace gasin the aforementioned mixed gas is higher than the molar concentrationof the second gas included in the trace gas in the mixed gas, thepermeance ratio (P40/P10) is higher than or equal to 0.77 and lower thanor equal to 1.00. In this case, it is possible to easily suppress atime-varying reduction in permeability of the zeolite membrane 12 in thesame manner as described above. Note that, as described above, the mixedgas includes the high permeability gas and the trace gas that is lowerin concentration than the high permeability gas. The first gas is a gaswhose adsorption equilibrium constant on the zeolite membrane 12 is lessthan 60 times the adsorption equilibrium constant of the highpermeability gas. The second gas is a gas whose adsorption equilibriumconstant on the zeolite membrane 12 is 400 times or more the adsorptionequilibrium constant of the high permeability gas.

As described above, the zeolite membrane 12 is in particular suitablefor use in the separation of the mixed gas when the high permeabilitygas is H₂, N₂, O₂, or CO₂.

As described above, the zeolite membrane 12 is suitable for use in theseparation of the high permeability gas from the low permeability gaswhen the mixed gas further includes a low permeability gas that ishigher in concentration than the trace gas.

The gas separation method described above may be modified in variousways.

For example, the step of removing the second gas from the mixed gas(step S12) may be omitted from the gas separation method describedabove.

The molar concentration of the first gas in the mixed gas may be lessthan 40 times the molar concentration of the second gas as long as themolar concentration of the first gas in the mixed gas is higher than themolar concentration of the second gas.

As described above, the mixed gas does not necessarily have to includethe low permeability gas as long as the mixed gas includes the highpermeability gas and the trace gas. The high permeability gas does notnecessarily have to be H₂, N₂, O₂, or CO₂, and may be a different gasother than these types of gas.

The maximum number of membered rings in the zeolite membrane 12 may besmaller than 8, or may be larger than 8.

The zeolite membrane complex 1 may further include a functional membraneor a protection membrane that is laminated on the zeolite membrane 12,in addition to the support 11 and the zeolite membrane 12. Thefunctional membrane or the protection membrane may be an inorganicmembrane such as a zeolite membrane, a silica membrane, or a carbonmembrane, or may be an organic membrane such as a polyimide membrane ora silicone membrane. Besides, a substance that has high absorbency forCO₂ may be added to the functional or protection membrane laminated onthe zeolite membrane 12.

The configurations of the above-described preferred embodiments andvariations may be appropriately combined as long as there are no mutualinconsistencies.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore to be understood that numerousmodifications and variations can be devised without departing from thescope of the invention.

INDUSTRIAL APPLICABILITY

The gas separation method according to the present invention isapplicable in various fields in which the separation of a gas isconducted.

REFERENCE SIGNS LIST

-   1 zeolite membrane complex-   11 support-   12 zeolite membrane-   S11 to S13 step

1. A gas separation method of separating a mixed gas, the gas separationmethod comprising: a) preparing a zeolite membrane complex that includesa porous support and a zeolite membrane formed on said support; and b)supplying a mixed gas that includes a high permeability gas and a tracegas to said zeolite membrane complex, the trace gas being lower inconcentration than said high permeability gas, and permeating said highpermeability gas through said zeolite membrane complex to separate saidhigh permeability gas from other gases, wherein a molar concentration ofa first gas included in said trace gas in said mixed gas is higher thana molar concentration of a second gas included in said trace gas in saidmixed gas, an adsorption equilibrium constant of said first gas on saidzeolite membrane being less than 60 times an adsorption equilibriumconstant of said high permeability gas on said zeolite membrane, and anadsorption equilibrium constant of said second gas on said zeolitemembrane being 400 times or more the adsorption equilibrium constant ofsaid high permeability gas in said zeolite membrane.
 2. The gasseparation method according to claim 1, wherein the molar concentrationof said first gas in said mixed gas is 40 times or more the molarconcentration of said second gas in said mixed gas.
 3. The gasseparation method according to claim 1, wherein said high permeabilitygas is hydrogen, nitrogen, oxygen, or carbon dioxide.
 4. The gasseparation method according to claim 1, wherein at least one of saidfirst gas and said second gas is an organic substance.
 5. The gasseparation method according to claim 1, wherein said zeolite membranecontains a zeolite crystal whose maximum number of membered rings is 8.6. The gas separation method according to claim 1, further comprising:before said operation b), removing said second gas from said mixed gas.7. The gas separation method according to claim 1, wherein said mixedgas further includes a low permeability gas to which said zeolitemembrane has lower permeability than to said high permeability gas, andin said mixed gas, said trace gas is lower in concentration than saidlow permeability gas.
 8. The gas separation method according to claim 1,wherein said mixed gas contains one or more types of substances selectedfrom among hydrogen, helium, nitrogen, oxygen, water, water vapor,carbon monoxide, carbon dioxide, nitrogen oxide, ammonia, sulfur oxide,hydrogen sulfide, sulfur fluoride, mercury, arsine, hydrogen cyanide,carbonyl sulfide, C₁ to C₈ hydrocarbons, organic acid, alcohol,mercaptans, ester, ether, ketone, and aldehyde.
 9. A zeolite membraneformed on a porous support, wherein when a mixed gas that includes ahigh permeability gas and a trace gas that is lower in concentrationthan said high permeability gas is supplied to said zeolite membrane,and in a state in which a molar concentration of a first gas included insaid trace gas in said mixed gas is higher than a molar concentration ofa second gas included in said trace gas in said mixed gas, a permeanceratio of P40/P10 is higher than or equal to 0.77 and lower than or equalto 1.00, an adsorption equilibrium constant of said first gas on saidzeolite membrane being less than 60 times an adsorption equilibriumconstant of said high permeability gas on said zeolite membrane, and anadsorption equilibrium constant of said second gas on said zeolitemembrane being 400 times or more the adsorption equilibrium constant ofsaid high permeability gas on said zeolite membrane.
 10. The zeolitemembrane according to claim 9, wherein said high permeability gas ishydrogen, nitrogen, oxygen, or carbon dioxide.
 11. The zeolite membraneaccording to claim 9, wherein said mixed gas further includes a lowpermeability gas to which said zeolite membrane has lower permeabilitythan to said high permeability gas, and in said mixed gas, said tracegas is lower in concentration said low permeability gas.